1. Field of the Invention
This invention relates to a process for decreasing the hygroscopicity of polycarboxylates and to the improvement of the fluidity of polycarboxylates.
2. Description of the Prior Art
Polycarboxylates used as polyelectrolytes are polymers whose characteristics are determined essentially by the presence of numerous carboxyl or carboxylate groups. Polycarboxylates are known to have a more or less strong tendency to adsorb water vapor from the air. This characteristic is particularly evident in polyelectrolytes in the form of alkali metal or ammonium salts. Under certain circumstances, this tendency to absorb water vapor is so strong that these substances are inclined to lump very quickly, become sticky or even to deliquesce. These phenomena even can occur when the substances are in an atmosphere of relatively low temperature, for example 20.degree. C, and relatively low relative humidity, for example 40 to 60%.
In order to decrease the hygroscopic characteristics of such polycarboxylates, it is known in the art to use the acid forms, that is, the free polycarboxylic acids, which are generally essentially less hygroscopic than the salts. For example, German published application No. 2,100,500 discloses a process for decreasing the hygroscopicity of polycarboxylates by a special production process; the polycarboxylates are produced by only partially neutralizing polycarboxylic acids with alkalis up to a neutralization degree of a maximum of 60%.
Other known processes for decreasing the undesirable consequences of hygroscopicity of polycarboxylates involve the use of waterproofed, finely dispersed silicic acids which keep the hygroscopic powder capable of flowing. (see for example German OS No. 2,134,695).
It is also known that the hygroscopicity of certain substances can be eliminated by encapsulating them in suitable casing materials or by subjecting them to a "microscopically" operative coating process, in analogy to "macroscopical" processes. The so-called "Prill process" is a particularly easy, modern coating technique. In that case, the hygroscopic substance is added as a powder to a melt of the casing material until a highly concentrated, fluid suspension of the hygroscopic substance forms, which is still sprayable through nozzles. This suspension is subsequently sprayed through spraying nozzles into a spray column, whereby the casing material still in a fluid state forms an almost precisely sperical casing around the powder particles of hygroscopic substance in each sprayed suspension droplet. While falling through the spray column, the sprayed droplets cool to such an extent that the casing material solidifies. Finally, a powder of spherical particles is obtained on the bottom of the column. This process has the disadvantage that relatively large quantities of casing material relative to the substance to be encased are required. For example, at least about 1 part by weight of casing material is employed for each 1 part by weight of substance to be encased. This is necessary in order to insure the fluidity of the suspension, its sprayability and the development of spherical particles.
Generally, processes for mixing hygroscopic substances with solid or fluid non-hygroscopic substances in different mixing systems, on the other hand, lead only to a decrease of hygroscopicity corresponding to the mixing ratio. There is no decisive influence on the characteristics of the hygroscopic substance (the decrease in hygroscopicity is expressed as a decrease of the volume of water absorbed at a certain temperature and at a certain relative humidity of air by the weight of the hygroscopic substance or of the mixture within a certain period). In accordance with the "snowballing" principle, every such mixing process will generally lead to a more or less quick agglomeration of the powder particles while forming crumbs up to a few centimeters, which are screened out or ground up after the mixing process.
Thus, there exists a need in the art for a process for decreasing the hygroscopicity of polycarboxylates. The process should be one in which a casing material is employed. Furthermore, the amount of casing material employed to obtain products of decreased hygroscopicity should be less than that required in prior art processes. The process should yield freely flowing, products.